The present invention relates to radio-frequency (rf) resonant cavities, and more particularly, to a cavity structure having generally planar members which cooperatively define multiple, serially aligned and coupled resonant cavities with a constant electric field gradient.
Radio-frequency, resonant cavity structures are found in charged particle accelerators, undulators, free electron lasers, oscillators, amplifier tubes and communication signal filters. As these devices continue to find wider application in various fields, such as medicine and industry, it is important to be able to manufacture resonant cavity structures which are compact and precise, yet easy and inexpensive to fabricate. This requirement is especially important in linear, charged particle accelerators, wherein hundreds or even thousands of resonant cavities, as well as the numerous accelerator structures containing the cavities, must be precisely fabricated and aligned.
In conventional accelerator structures, resonant cavities are generally cylindrical in shape and require extensive machining, as well as subsequent assembly and alignment. However, for resonant cavities which operate at high frequencies (wavelengths of only a few millimeters), the fabrication of these cylindrical structures by conventional machining and brazing methods is extremely difficult and expensive because of the precise tolerances and specifications that are required. Additionally, because the cavities are normally aligned and joined by standard welding and brazing methods, frequent tunings and adjustments are necessary. These problems become even more exacerbated in applications such as linear accelerators, wherein hundreds or thousands of identical and precise resonant cavities must be manufactured and aligned.
One method of producing inexpensive and precise resonant cavities is to mass produce them using deep x-ray lithography (DXL) microfabrication techniques such as Lithography Galvanoformung Abformung (LIGA). In this method, x-rays are used to expose a predetermined pattern on a relatively thick layer of resist such as poly(methylmethacrylate), PMMA, which overlays a substrate. Next, after the areas of the PMMA which have been exposed are removed, metal is electroplated into the vacated areas. Then, the remaining PMMA is removed to leave a generally planar member having indentations or hollows of a predetermined pattern. A plurality of these members are then engaged to produce an accelerator structure having multiple resonant cavities.
The advantages of manufacturing such planar accelerator structures is that they can be mass produced with excellent precision, as the uniformity of the hollows can be within a few hundred angstroms. The structures do not require extensive assembly or tuning, and the materials from which the accelerator structure can be made is more diverse since no machining is involved.
In FIGS. 1(a), 1(b), and 1(c) is shown a generally planar housing member 10 (see FIG. 1(a)) produced by the microfabrication technique as described above. The member 10 has a plurality of rectangularly shaped hollows 11, which are positioned in a linear, serial fashion. In FIGS. 2(a), 2(b), and (c) is shown an accelerator structure 20 formed by placing two members 10, using spacers 22 for alignment (best seen in FIG. 2(a)), in such a manner so as to allow the hollows 11 see FIG. 2(b) and 2(c) to cooperatively form generally cubically shaped resonant cavities 21 (see FIG. 2(b)). The cavities 21, when subjected to rf waves, are coupled so as to provide acceleration to a charged particle traveling generally along a longitudinal axis through the cavities 21.
The accelerator structure 20 shown in FIGS. 2(a), 2(b), and (c) is of the constant impedance type, wherein the electric field gradient within the structure 20 varies along the longitudinal axis of the structure 20. Because of its uniform pattern, such a structure 20 is simple to fabricate, but is not the most optimal for accelerator applications, because of heat loading at the rf input stages which limits the total power that can be applied to the accelerator structure 20.
For accelerator applications, constant field gradient type structures have been preferred because of its higher energy gain and better frequency characteristics. Such structures have more uniform power dissipation, higher shunt impedance and is less sensitive to frequency deviations and beam break-up when compared to comparable constant impedance structures.
In conventional, circularly cylindrical accelerator structures, a constant gradient structure can be achieved by varying the dimensions of the resonant cavities and the coupling apertures. However, it is more difficult to construct a constant gradient structure using the DXL lithographic techniques as described above, since the depth of the hollows 11 created by such processing will be identical and normally cannot be varied. Using lithographic processing, cavity 21 dimensions can be varied by changing the width and length, or shapes, of the hollows 11. However, simulations have shown that such shape changes alone are not sufficient to produce good cavity coupling at constant operating frequencies and are unsuitable for achieving high accelerating voltages.
A simple method of varying the coupling, or the vertical dimensions, of the cavities 21 is to positions the members 10 in such a fashion so as to vary the distance between two members 10 as shown in FIG. 3. In this method, the vertical distance, D, between the members is varied along a beam direction, z, such that D is made increasingly smaller along the beam direction, z in an axis, y, perpendicular to the z direction. This structure 30, as shown in FIG. 3, is able to produce a field gradient which is somewhat more constant than the simple structure shown in FIG. 2. However, this structure 30 is difficult to align correctly, especially when constructing multiple member structures, and is not very efficient in producing high field gradients.
In view of the foregoing, the general object of this invention is to provide a rf cavity structure which is cheap and easy to manufacture and yet is efficient and can achieve high operating voltages and constant field gradients.
Another object of this invention is to provide a cavity structure having generally planar members with hollow defined therein which cooperate to form resonant cavities.
Yet another object of this invention is to provide a cavity structure that is produced by DXL microfabrication methods.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.